Body copy has never been a significant factor at Google in terms of ranking but is absolutely critical to long-tail ranking where no-one is generally even trying to rank, so just having the phrase somewhere on the page is sufficient to get as much traffic as there is.
Each of these long-tail searches is really small in volume but they are more numerous than we will ever actually measure so the total can generally add up to be half of all the traffic to you site.
Because the strength of body copy is already so low, variations within copy such as header tags, bolding of words and the like are simply too tedious to measure and frankly not worth the trouble anyway – but that doesn’t mean I don’t religiously follow-through on header tags because frankly a lot of SEO is tedious, but it lends significantly to your armoury of discipline and focus if you can face-down the imponderables and flex every keyphrase muscle within reach.
On the other hand, if you are trying to outrank somebody with your body copy, you’re just so missing the boat: use some links and a better title instead.
For anything you intend to rank for and put in the effort to rank for, you should only ever be using link text, page rank and the title tag; body copy is for things you’re accidentally ranking for. The ranking you get from body copy is a gift, but is a gift that keeps on giving by (oh, there’s a catch, always) writing, adding more copy and broader-focussed copy that will attract a greater variety of searches.
Suffice to say, you really need developed and developing body copy to harvest that 50% of search that you can’t really plan for.
That said, let’s move on to getting searchers to click the rankings that you do get.
Some On-Page LinkLuv
On-Page SEO: Hello World!
On-Page SEO: the Title tag
On-Page SEO – Keyphrase Positioning – First Fit v. Best Fit
On-Page SEO – Keyphrase Positioning – Display Order v. Markup Order
On-Page SEO – Head and Body – Both Feet in Both Camps
Related: String Theory – How Many Characters in a Page Title for Google SEO?